Boston Marathon Bombing: A Look Back

WATERTOWN, MA - APRIL 19: Police officers and SWAT team members celebrate after the successful operation to capture 19-year-old bombing suspect Dzhokhar A. Tsarnaev on April 19, 2013 in Watertown, Massachusetts. A manhunt for a suspect in the Boston Marathon bombing, Dzhokhar A. Tsarnaev, 19, ended this evening with his capture on a boat parked on a residential property in Watertown, Massachusetts. His brother Tamerlan Tsarnaev, 26, the other suspect, was shot and killed by police early this morning after a car chase and shootout with police. The two men are suspects in the bombings at the Boston Marathon on April 15 that killed three people and wounded at least 170. (Photo by Mario Tama/Getty Images)

US-ATTACKS-BOSTON

Boston Swat team members smile after the capture of the second of two suspects wanted in the Boston Marathon bombings April 19, 2013 in Watertown, Massachusetts. Thousands of heavily armed police staged an intense manhunt for a Chechen teenager suspected in the Boston marathon bombings with his brother, who was killed in a shootout. Dzhokhar Tsarnaev, 19, defied the massive force after his 26-year-old brother Tamerlan was shot and suffered critical injuries from explosives believed to have been strapped to his body. AFP PHOTO / TIMOTHY A. CLARY (Photo credit should read TIMOTHY A. CLARY/AFP/Getty Images)

Shootings In Cambridge, Watertown Draw Massive Police Response

<> on April 19, 2013 in Cambridge, Massachusetts.

Shootings In Cambridge, Watertown Draw Massive Police Response

<> on April 19, 2013 in Cambridge, Massachusetts.

Shootings In Cambridge, Watertown Draw Massive Police Response

WATERTOWN, MA - APRIL 19: Many different law enforcement agencies descend on an area around Franklin Street on April 19, 2013 in Watertown, Massachusetts. After a car chase and shootout with police, one suspect in the Boston Marathon bombing, Tamerlan Tsarnaev, 26, was shot and killed by police early morning April 19, and a manhunt is underway for his brother and second suspect, 19-year-old Dzhokhar A. Tsarnaev. The two men are suspects in the bombings at the Boston Marathon on April 15 that killed three people and wounded at least 170. (Photo by Jared Wickerham/Getty Images)

US-ATTACKS-BOSTON

Residents are pictured after the capture of the second of two suspects wanted in the Boston Marathon bombings April 19, 2013 in Watertown, Massachusetts. Thousands of heavily armed police staged an intense manhunt for a Chechen teenager suspected in the Boston marathon bombings with his brother, who was killed in a shootout. Dzhokhar Tsarnaev, 19, defied the massive force after his 26-year-old brother Tamerlan was shot and suffered critical injuries from explosives believed to have been strapped to his body. AFP PHOTO / TIMOTHY A. CLARY (Photo credit should read TIMOTHY A. CLARY/AFP/Getty Images)

US-ATTACKS-BOSTON

Boston Swat team members smile after the capture of the second of two suspects wanted in the Boston Marathon bombings April 19, 2013 in Watertown, Massachusetts. Thousands of heavily armed police staged an intense manhunt for a Chechen teenager suspected in the Boston marathon bombings with his brother, who was killed in a shootout. Dzhokhar Tsarnaev, 19, defied the massive force after his 26-year-old brother Tamerlan was shot and suffered critical injuries from explosives believed to have been strapped to his body. AFP PHOTO / TIMOTHY A. CLARY (Photo credit should read TIMOTHY A. CLARY/AFP/Getty Images)

Shootings In Cambridge, Watertown Draw Massive Police Response

<> on April 19, 2013 in Cambridge, Massachusetts.

Shootings In Cambridge, Watertown Draw Massive Police Response

WATERTOWN, MA - APRIL 19: Law enforcement evacuate people near to where a suspect is hiding on Franklin St., April 19, 2013 in Watertown, Massachusetts. After a car chase and shoot out with police, one suspect in the Boston Marathon bombing, Tamerlan Tsarnaev, 26, was shot and killed by police early morning April 19, and a manhunt is underway for his brother and second suspect, 19-year-old Dzhokhar A. Tsarnaev. The two men are suspects in the bombings at the Boston Marathon on April 15, that killed three people and wounded at least 170. (Photo by Darren McCollester/Getty Images)

US-ATTACKS-BOSTON

The Cape Cod, Massachusetts, Police Department SWAT team search houses for the second of two suspects wanted in the Boston Marathon bombings takes place April 19, 2013 in Watertown, Massachusetts. Thousands of heavily armed police staged an intense manhunt Friday for a Chechen teenager suspected in the Boston marathon bombings with his brother, who was killed in a shootout. Dzhokhar Tsarnaev, 19, defied the massive force after his 26-year-old brother Tamerlan was shot and suffered critical injuries from explosives believed to have been strapped to his body. AFP PHOTO / TIMOTHY A. CLARY (Photo credit should read TIMOTHY A. CLARY/AFP/Getty Images)

Boston Marathon Explosions

Nearly 200 runners have been hurt and three were killed in two explosions near the Boston Marathon finish line. (credit: WBZ-TV)

Boston Marathon Explosions

Nearly 200 runners have been hurt and three were killed in two explosions near the Boston Marathon finish line. (credit: WBZ-TV)

Boston Marathon Explosions

Nearly 200 runners have been hurt and three were killed in two explosions near the Boston Marathon finish line. (credit: WBZ-TV)

Boston Marathon Explosions

Nearly 200 runners have been hurt and three were killed in two explosions near the Boston Marathon finish line. (credit: WBZ-TV)

Boston Marathon Explosions

Nearly 200 runners have been hurt and three were killed in two explosions near the Boston Marathon finish line. (credit: WBZ-TV)

Multiple People Injured After Explosions Near Finish Line at Boston Marathon

BOSTON, MA - APRIL 15: A runner sits near Kenmore Square after two bombs exploded during the 117th Boston Marathon on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon. (Photo by Alex Trautwig/Getty Images)

Boston Marathon Explosions

Explosions near the Boston Marathon finish line. (credit: WBZ-TV)

Multiple People Injured After Explosions Near Finish Line at Boston Marathon

BOSTON, MA - APRIL 15: The bandaged leg of a man hangs outside an ambulance outside a medical tent located ear the finish of the 117th Boston Marathon after two bombs exploded on the marathon route on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon. (Photo by Jim Rogash/Getty Images)

Multiple People Injured After Explosions Near Finish Line at Boston Marathon

BOSTON, MA - APRIL 15: A man is loaded into an ambulance after he was injured by one of two bombs exploded during the 117th Boston Marathon near Copley Square on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon. (Photo by Jim Rogash/Getty Images)

Multiple People Injured After Explosions Near Finish Line at Boston Marathon

A man is loaded into an ambulance after he was injured by one of two bombs exploded during the 117th Boston Marathon near Copley Square on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon. (Photo: Jim Rogash/Getty Images)

Multiple People Injured After Explosions Near Finish Line at Boston Marathon

BOSTON, MA - APRIL 15: A man is loaded into an ambulance after he was injured by one of two bombs exploded during the 117th Boston Marathon near Copley Square on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon. (Photo by Jim Rogash/Getty Images)

Multiple People Injured After Explosions Near Finish Line at Boston Marathon

BOSTON, MA - APRIL 15: A man is loaded into an ambulance after he was injured by one of two bombs exploded during the 117th Boston Marathon near Copley Square on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon. (Photo by Jim Rogash/Getty Images)

Multiple People Injured After Explosions Near Finish Line at Boston Marathon

BOSTON, MA - APRIL 15: A woman is loaded into an ambulance after he was injured by one of two bombs exploded during the 117th Boston Marathon near Copley Square on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon. (Photo by Jim Rogash/Getty Images)

Multiple People Injured After Explosions Near Finish Line at Boston Marathon

BOSTON, MA - APRIL 15: A member of the bomb squad investigates a suspicious item on the road near Kenmore Square after two bombs exploded during the 117th Boston Marathon on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon. (Photo by Alex Trautwig/Getty Images)

Multiple People Injured After Explosions Near Finish Line at Boston Marathon

BOSTON, MA - APRIL 15: State police guard an area near Kenmore Square after two bombs exploded during the 117th Boston Marathon on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon. (Photo by Alex Trautwig/Getty Images)

Multiple People Injured After Explosions Near Finish Line at Boston Marathon

BOSTON, MA - APRIL 15: A member of the bomb squad investigates a suspicious item on the road near Kenmore Square after two bombs exploded during the 117th Boston Marathon on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon. (Photo by Alex Trautwig/Getty Images)

Multiple People Injured After Explosions Near Finish Line at Boston Marathon

BOSTON, MA - APRIL 15: A member of the bomb squad investigates a suspicious item on the road near Kenmore Square after two bombs exploded during the 117th Boston Marathon on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon. (Photo by Alex Trautwig/Getty Images)

Multiple People Injured After Explosions Near Finish Line at Boston Marathon

BOSTON, MA - APRIL 15: A runner reacts near Kenmore Square after two bombs exploded during the 117th Boston Marathon on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon. (Photo by Alex Trautwig/Getty Images)

Multiple People Injured After Explosions Near Finish Line at Boston Marathon

BOSTON, MA - APRIL 15: A runner embraces another woman on the marathon route near Kenmore Square after two bombs exploded during the 117th Boston Marathon on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon. (Photo by Alex Trautwig/Getty Images)

Multiple People Injured After Explosions Near Finish Line at Boston Marathon

BOSTON, MA - APRIL 15: A runner embraces another woman near Kenmore Square after two bombs exploded during the 117th Boston Marathon on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon. (Photo by Alex Trautwig/Getty Images)

Multiple People Injured After Explosions Near Finish Line at Boston Marathon

BOSTON, MA - APRIL 15: Members of the bomb squad investigate a suspicious item on the road as runners pass near Kenmore Square after two bombs exploded during the 117th Boston Marathon on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon. (Photo by Alex Trautwig/Getty Images)

Multiple People Injured After Explosions Near Finish Line at Boston Marathon

BOSTON, MA - APRIL 15: Two blood stained feet of a man hangs outside an ambulance outside a medical tent located near the finish of the 117th Boston Marathon after two bombs exploded on the marathon route on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon. (Photo by Jim Rogash/Getty Images)

Multiple People Injured After Explosions Near Finish Line at Boston Marathon

BOSTON, MA - APRIL 15: Two blood stained feet of a man hang outside an ambulance outside a medical tent located near the finish of the 117th Boston Marathon after two bombs exploded on the marathon route on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon. (Photo by Jim Rogash/Getty Images)

Multiple People Injured After Explosions Near Finish Line at Boston Marathon

BOSTON, MA - APRIL 15: The bandaged leg of a man hangs outside an ambulance outside a medical tent located ear the finish of the 117th Boston Marathon after two bombs exploded on the marathon route on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon. (Photo by Jim Rogash/Getty Images)

Multiple People Injured After Explosions Near Finish Line at Boston Marathon

BOSTON, MA - APRIL 15: A woman is loaded into an ambulance after he was injured by one of two bombs exploded during the 117th Boston Marathon near Copley Square on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon. (Photo by Jim Rogash/Getty Images)

Multiple People Injured After Explosions Near Finish Line at Boston Marathon

BOSTON, MA - APRIL 15: A woman is loaded into an ambulance after he was injured by one of two bombs exploded during the 117th Boston Marathon near Copley Square on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon. (Photo by Jim Rogash/Getty Images)

Multiple People Injured After Explosions Near Finish Line at Boston Marathon

BOSTON, MA - APRIL 15: A woman, with blood on fingers, is loaded into an ambulance after being injured after two bombs exploded on the marathon route on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon. (Photo by Jim Rogash/Getty Images)

Multiple People Injured After Explosions Near Finish Line at Boston Marathon

BOSTON, MA - APRIL 15: A woman looks at the blood on her hands as she is loaded into an ambulance after being injured after two bombs exploded on the marathon route on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon. (Photo by Jim Rogash/Getty Images)

US-ATHLETICS-MARATHON-BLAST

A long line of ambulances wait in a staging area after explosions rocked the finish area of the Boston Marathon on April 15, 2013 in Boston, Massachusetts. At least two people were killed and 22 wounded when two explosions struck near the finish line of the Boston Marathon, sparking scenes of panic, police said. The streets were littered with debris and blood and paramedics raced off with stretchers as police locked down the area, witness said. TV footage showed an explosion sending up a white plume of smoke along the sidelines of the race. AFP PHOTO/John MOTTERN (Photo credit should read JOHN MOTTERN/AFP/Getty Images)

Running Of The 117th Boston Marathon

BOSTON, MA - APRIL 15: Gebregziabher Gebremariam of Ethiopia is helped after he stumbled over the finsih line to finish second in the 117th Boston Marathon on April 15, 2013 in Boston, Massachusetts. (Photo by Jim Rogash/Getty Images)

US-MARATHON-BLAST

Boston police block off the area around the finish of the Boston Marathon as they move spectators and runners from the scene were several explosions rocked the finish of the race on April 15, 2013 in Boston, Massachusetts. At least two people were killed and 22 wounded when two explosions struck near the finish line of the Boston Marathon, sparking scenes of panic, police said. The streets were littered with debris and blood and paramedics raced off with stretchers as police locked down the area, witness said. TV footage showed an explosion sending up a white plume of smoke along the sidelines of the race. AFP PHOTO/John MOTTERN (Photo credit should read JOHN MOTTERN/AFP/Getty Images)

Multiple People Injured After Explosions Near Finish Line at Boston Marathon

Firefighters take postion on Boyltson Street near the finish line after two bombs exploded during the 117th Boston Marathon on April 15, 2013 in Boston, Massachusetts. (Photo by Darren McCollester/Getty Images)

BOSTON, MA - APRIL 15: Firefighters take postion on Boyltson Street near the finish line after two bombs exploded during the 117th Boston Marathon on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 28 injured after at least two explosions went off near the finish line to the marathon. (Photo by Darren McCollester/Getty Images)

Multiple People Injured After Explosions Near Finish Line at Boston Marathon

ce and runners stand near Kenmore Square after two bombs exploded during the 117th Boston Marathon (Photo by Alex Trautwig/Getty Images)

BOSTON, MA - APRIL 15: Police and runners stand near Kenmore Square after two bombs exploded during the 117th Boston Marathon on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 28 injured after at least two explosions went off near the finish line to the marathon. (Photo by Alex Trautwig/Getty Images)

Multiple People Injured After Explosions Near Finish Line at Boston Marathon

BOSTON, MA - APRIL 15: People react to a suspicious package on the corner of Dartmouth and Stuart St after two bombs exploded during the 117th Boston Marathon on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 28 injured after at least two explosions went off near the finish line to the marathon. (Photo by Darren McCollester/Getty Images)

Multiple People Injured After Explosions Near Finish Line at Boston Marathon

eople walk near Kenmore Square after two bombs exploded during the 117th Boston Marathon on April 15, 2013 in Boston, Massachusetts. (Photo by Alex Trautwig/Getty Images)

BOSTON, MA - APRIL 15: People walk near Kenmore Square after two bombs exploded during the 117th Boston Marathon on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon. (Photo by Alex Trautwig/Getty Images)

Multiple People Injured After Explosions Near Finish Line at Boston Marathon

(Credit: Alex Trautwig/Getty Images)

BOSTON, MA - APRIL 15: Runners gather near Kenmore Square after two bombs exploded during the 117th Boston Marathon on April 15, 2013 in Boston, Massachusetts. Two people are confirmed dead and at least 23 injured after two explosions went off near the finish line to the marathon. (Photo by Alex Trautwig/Getty Images)

Boston Marathon Bombings

Flowers and a message are left on Newbury Street April 16, 2013 in Boston, Massachusetts, a few blocks from where two explosions struck near the finish line of the Boston Marathon on Monday. Three people, including a child, were killed and more than 170 were injured in the explosions that occurred a few seconds apart near the finish line of the 117th rendition of the world's oldest international marathon. (Photo credit: STAN HONDA/AFP/Getty Images)

Boston Marathon Bombings

Angelina Ciccantelli (L) and Whitney Kleinert lay flowers at a memorial site at Boylston and Arlington streets along the course of the Boston Marathon on April 16, 2013. The explosives used in the Boston Marathon bombings were likely homemade devices full of nails and metal fragments designed to cause widespread injury, according to initial reports. A day after an attack that left three dead and more than 170 wounded, the FBI and Boston police declined to reveal details of their probe, or whether they suspected the assault was linked to foreign or domestic extremists. (Photo credit: DON EMMERT/AFP/Getty Images)

Boston Marathon Bombings

Flowers and a message were left at Boylston and Arlington streets on April 16, 2013 in Boston, a few blocks from where two explosions struck near the finish line of the Boston Marathon on April 15. The explosives used in the Boston Marathon bombings were likely homemade devices full of nails and metal fragments designed to cause widespread injury, according to initial reports. A day after an attack that left three dead and more than 170 wounded, the FBI and Boston police declined to reveal details of their probe, or whether they suspected the assault was linked to foreign or domestic extremists. (Photo credit: STAN HONDA/AFP/Getty Images)

Boston Marathon Bombings

A Boston police officer stands guard at a memorial site at Boylston and Arlington streets along the course of the Boston Marathon on April 16, 2013, a few blocks from where two explosions struck near the finish line of the Boston Marathon on April 15. The explosives used in the Boston Marathon bombings were likely homemade devices full of nails and metal fragments designed to cause widespread injury, according to initial reports. A day after an attack that left three dead and more than 170 wounded, the FBI and Boston police declined to reveal details of their probe, or whether they suspected the assault was linked to foreign or domestic extremists. (Photo credit: DON EMMERT/AFP/Getty Images)

A Boston police officer stands guard at a memorial site at Boylston and Arlington streets along the course of the Boston Marathon on April 16, 2013, a few blocks from where two explosions struck near the finish line of the Boston Marathon on April 15. The explosives used in the Boston Marathon bombings were likely homemade devices full of nails and metal fragments designed to cause widespread injury, according to initial reports. A day after an attack that left three dead and more than 170 wounded, the FBI and Boston police declined to reveal details of their probe, or whether they suspected the assault was linked to foreign or domestic extremists. (Photo credit: DON EMMERT/AFP/Getty Images)

Boston Marathon Bombings

A tube of Bengay, a pain relief medecine, lays nears flowers at a memorial site at Boylston and Arlington streets along the course of the Boston Marathon on April 16, 2013, a few blocks from where two explosions struck near the finish line of the Boston Marathon on April 15. The explosives used in the Boston Marathon bombings were likely homemade devices full of nails and metal fragments designed to cause widespread injury, according to initial reports. A day after an attack that left three dead and more than 170 wounded, the FBI and Boston police declined to reveal details of their probe, or whether they suspected the assault was linked to foreign or domestic extremists. (Photo credit: DON EMMERT/AFP/Getty Images)

Boston Marathon Bombings

People hold candles during a vigil on the Boston Common on April 16, 2013 in Boston, in the aftermath of two explosions that struck near the finish line of the Boston Marathon on April 15. Investigators said the range of suspects and motives in the grisly Boston bombings remained "wide open" as experts assessed remnants of the crude devices designed to inflict maximum suffering. (Photo credit:STAN HONDA/AFP/Getty Images)

Boston Marathon Bombings

BOSTON, MA - APRIL 16: People attend a vigil for victims of the Boston Marathon bombings at Boston Commons on April 16, 2013 in Boston, Massachusetts. The twin bombings, which occurred near the marathon finish line, resulted in the deaths of three people while hospitalizing at least 140. The bombings at the 116-year-old Boston race, resulted in heightened security across the nation with cancellations of many professional sporting events as authorities search for a motive to the violence. (Photo credit: Spencer Platt/Getty Images)

Boston Marathon Bombings

BOSTON, MA - APRIL 16: A couple hugs during a vigil for victims of the Boston Marathon bombings at Boston Commons on April 16, 2013 in Boston, Massachusetts. The twin bombings, which occurred near the marathon finish line, resulted in the deaths of three people while hospitalizing at least 140. The bombings at the 116-year-old Boston race, resulted in heightened security across the nation with cancellations of many professional sporting events as authorities search for a motive to the violence. (Photo credit: Spencer Platt/Getty Images)

Boston Marathon Bombings

People gather in the Boston Commons for a candlelight vigil on April 16, 2013 in Boston, in the aftermath of two explosions that struck near the finish line of the Boston Marathon on April 15. Investigators said the range of suspects and motives in the grisly Boston bombings remained "wide open" as experts assessed remnants of the crude devices designed to inflict maximum suffering. AFP PHOTO/Don Emmert (Photo credit: DON EMMERT/AFP/Getty Images)

Boston Marathon Bombings

BOSTON, MA - APRIL 16: A couple hugs during a vigil for victims of the Boston Marathon bombings at Boston Commons on April 16, 2013 in Boston, Massachusetts. The twin bombings, which occurred near the marathon finish line, resulted in the deaths of three people while hospitalizing at least 140. The bombings at the 116-year-old Boston race, resulted in heightened security across the nation with cancellations of many professional sporting events as authorities search for a motive to the violence. (Photo credit: Spencer Platt/Getty Images)

Boston Marathon Bombings

People sign messages on a banner during a vigil on the Boston Common on April 16, 2013 in Boston, in the aftermath of two explosions that struck near the finish line of the Boston Marathon on April 15. Investigators said the range of suspects and motives in the grisly Boston bombings remained "wide open" as experts assessed remnants of the crude devices designed to inflict maximum suffering. (Photo credit: STAN HONDA/AFP/Getty Images)

Boston Marathon Bombings

Messages on a banner are seen during a vigil on the Boston Common on April 16, 2013 in Boston, in the aftermath of two explosions that struck near the finish line of the Boston Marathon on April 15. Investigators said the range of suspects and motives in the grisly Boston bombings remained "wide open" as experts assessed remnants of the crude devices designed to inflict maximum suffering. AFP PHOTO/Stan HONDA (Photo credit: STAN HONDA/AFP/Getty Images)

Boston Marathon Bombings

A message written on a banner seen during a vigil on the Boston Common on April 16, 2013 in Boston, in the aftermath of two explosions that struck near the finish line of the Boston Marathon on April 15. Investigators said the range of suspects and motives in the grisly Boston bombings remained "wide open" as experts assessed remnants of the crude devices designed to inflict maximum suffering. (Photo credit: STAN HONDA/AFP/Getty Images)

Boston Marathon Bombings

People sign a poster as people gather in the Boston Commons for a vigil on April 16, 2013 in Boston, in the aftermath of two explosions that struck near the finish line of the Boston Marathon on April 15. Investigators said the range of suspects and motives in the grisly Boston bombings remained "wide open" as experts assessed remnants of the crude devices designed to inflict maximum suffering. (Photo credit: DON EMMERT/AFP/Getty Images)

Boston Marathon Bombings

BOSTON, MA - APRIL 16: A woman writes a message on a memorial canvas during a vigil for victims of the Boston Marathon bombings at Boston Commons on April 16, 2013 in Boston, Massachusetts. The twin bombings, which occurred near the marathon finish line, resulted in the deaths of three people while hospitalizing at least 140. The bombings at the 116-year-old Boston race, resulted in heightened security across the nation with cancellations of many professional sporting events as authorities search for a motive to the violence. (Photo credit: Spencer Platt/Getty Images)

Boston Marathon Bombings

BOSTON, MA - APRIL 16: People write messages on a memorial canvas during a vigil for victims of the Boston Marathon bombings at Boston Commons on April 16, 2013 in Boston, Massachusetts. The twin bombings, which occurred near the marathon finish line, resulted in the deaths of three people while hospitalizing at least 140. The bombings at the 116-year-old Boston race, resulted in heightened security across the nation with cancellations of many professional sporting events as authorities search for a motive to the violence. (Photo credit: Spencer Platt/Getty Images)

Boston Marathon Bombings

Students from the Clifden Academy hold an American flag and candles during a vigil ceremony April 16, 2013 in Dorcester, Massachusetts, honoring the Richard family, who's 8-year-old son Martin was killed, and sister Jane, who lost a leg and had danced at the school, and mother Denise, who was also seriously injured when bombs exploded at the finish of the Boston Marathon April 15th. (Photo credit: JOHN MOTTERN/AFP/Getty Images)

Boston Marathon Bombings

BOSTON, MA - APRIL 16: A mother kisses her daughter on the head during a vigil for eight-year-old Martin Richard, from Dorchester, who was killed by an explosion near the finish line of the Boston Marathon on April 16, 2013 at Garvey Park in Boston, Massachusetts. The twin bombings resulted in the deaths of three people and hospitalized at least 128. The bombings at the 116-year-old Boston race resulted in heightened security across the nation with cancellations of many professional sporting events as authorities search for a motive to the violence. (Photo credit: Jared Wickerham/Getty Images)

Boston Marathon Bombings

BOSTON, MA - APRIL 16: A young boy holds a lit candle and an American flag atop his father's shoulders during a vigil for eight-year-old Martin Richard, from Dorchester, who was killed by an explosion near the finish line of the Boston Marathon on April 16, 2013 at Garvey Park in Boston, Massachusetts. The twin bombings resulted in the deaths of three people and hospitalized at least 128. The bombings at the 116-year-old Boston race resulted in heightened security across the nation with cancellations of many professional sporting events as authorities search for a motive to the violence. (Photo credit: Jared Wickerham/Getty Images)

Boston Marathon Bombings

BOSTON, MA - APRIL 16: Twin sisters hold a candle while listening during the vigil for eight-year-old Martin Richard, from Dorchester, who was killed by an explosion near the finish line of the Boston Marathon on April 16, 2013 at Garvey Park in Boston, Massachusetts. The twin bombings resulted in the deaths of three people and hospitalized at least 128. The bombings at the 116-year-old Boston race resulted in heightened security across the nation with cancellations of many professional sporting events as authorities search for a motive to the violence. (Photo credit: Jared Wickerham/Getty Images)

Boston Marathon Bombings

BOSTON, MA - APRIL 16: A group of students embrace during a vigil for eight-year-old Martin Richard, from Dorchester, who was killed by an explosion near the finish line of the Boston Marathon on April 16, 2013 at Garvey Park in Boston, Massachusetts. The twin bombings resulted in the deaths of three people and hospitalized at least 128. The bombings at the 116-year-old Boston race resulted in heightened security across the nation with cancellations of many professional sporting events as authorities search for a motive to the violence. (Photo credit: Jared Wickerham/Getty Images)

Boston Marathon Bombings

BOSTON, MA - APRIL 16: Young children stand with lit candles during a vigil for eight-year-old Martin Richard, from Dorchester, who was killed by an explosion near the finish line of the Boston Marathon on April 16, 2013 at Garvey Park in Boston, Massachusetts. The twin bombings resulted in the deaths of three people and hospitalized at least 128. The bombings at the 116-year-old Boston race resulted in heightened security across the nation with cancellations of many professional sporting events as authorities search for a motive to the violence. (Photo credit: Jared Wickerham/Getty Images)

Boston Marathon Bombings

BOSTON, MA - APRIL 16: A man with an American flag on his hat attends a vigil for eight-year-old Martin Richard, from Dorchester, who was killed by an explosion near the finish line of the Boston Marathon on April 16, 2013 at Garvey Park in Boston, Massachusetts. The twin bombings resulted in the deaths of three people and hospitalized at least 128. The bombings at the 116-year-old Boston race resulted in heightened security across the nation with cancellations of many professional sporting events as authorities search for a motive to the violence. (Photo credit: Jared Wickerham/Getty Images)

Boston Marathon Bombings

(Credit: Jared Wickerham/Getty Images)

BOSTON, MA - APRIL 16: A young girl cries with her mother during the vigil for eight-year-old Martin Richard, from Dorchester, who was killed by an explosion near the finish line of the Boston Marathon on April 16, 2013 at Garvey Park in Boston, Massachusetts. The twin bombings resulted in the deaths of three people and hospitalized at least 128. The bombings at the 116-year-old Boston race resulted in heightened security across the nation with cancellations of many professional sporting events as authorities search for a motive to the violence. (Photo credit: Jared Wickerham/Getty Images)

Boston Marathon Bombings

BOSTON, MA - APRIL 16: People gather with candles during a vigil for eight-year-old Martin Richard, from Dorchester, who was killed by an explosion near the finish line of the Boston Marathon on April 16, 2013 at Garvey Park in Boston, Massachusetts. The twin bombings resulted in the deaths of three people and hospitalized at least 128. The bombings at the 116-year-old Boston race resulted in heightened security across the nation with cancellations of many professional sporting events as authorities search for a motive to the violence. (Photo credit: Jared Wickerham/Getty Images)

Boston Deals With Aftermath Of Marathon Explosions

BOSTON, MA - APRIL 16: Unused thermal blankets for marathon participants are piled near the scene of a twin bombing at the Boston Marathon on April 16, 2013 in Boston, Massachusetts. The twin bombings, which occurred near the marathon finish line, resulted in the deaths of three people while hospitalizing at least 128. The bombings at the 116-year-old Boston race, resulted in heightened security across the nation with cancellations of many professional sporting events as authorities search for a motive to the violence. (Photo credit: Spencer Platt/Getty Images)

Security Remains Heightened In Washington Day After Bombings During Boston Marathon

(Photo credit: MANDEL NGAN/AFP/Getty Images)

The flag above the White House flies at half staff on April 16, 2013 in Washington, DC. At least three people were killed and over 100 wounded when two explosions struck near the finish line of the Boston Marathon on April 15. As a mark of respect for the victims of the senseless acts of violence perpetrated on April 15, 2013, in Boston, Massachusetts, US President Barack Obama issued a proclamation that the flag of the United States shall be flown at half-staff at the White House and upon all public buildings and grounds, at all military posts and naval stations, and on all naval vessels of the Federal Government in the District of Columbia and throughout the United States and its Territories and possessions until sunset, April 20, 2013. (Photo credit: MANDEL NGAN/AFP/Getty Images)

Security Remains Heightened In Washington Day After Bombings During Boston Marathon

Pedestrians pass next to a newspaper vending machine offering The Washington Post featuring the aftermath of the Boston bombings on its front page in Washington DC, April 16, 2013. US President Barack Obama branded the Boston bombings a "cowardly" act of terror, but said it was still unclear if a foreign or domestic group or individual was behind the attacks. (Photo credit: MLADEN ANTONOV/AFP/Getty Images)

Security Remains Heightened In Washington Day After Bombings During Boston Marathon

WASHINGTON, DC - APRIL 16: A U.S. Capitol Police K-9 team works at the entrance of the Capitol South Metro station during rush hour near the U.S. Captiol April 16, 2013 in Washington, DC. Police and other law enforcement agencies in Washington and across the country continue to operate at a heightened state of security after yesterday's Boston Marathon bombings. (Photo credit: Chip Somodevilla/Getty Images)

Security Remains Heightened In Washington Day After Bombings During Boston Marathon

WASHINGTON, DC - APRIL 16: A U.S. Capitol Police K-9 team works at the entrance of the Capitol South Metro station during rush hour near the U.S. Captiol April 16, 2013 in Washington, DC. Police and other law enforcement agencies in Washington and across the country continue to operate at a heightened state of security after yesterday's Boston Marathon bombings. (Photo credit: Chip Somodevilla/Getty Images)

Security Remains Heightened In Washington Day After Bombings During Boston Marathon

WASHINGTON, DC - APRIL 16: A U.S. Capitol Police K-9 team works at the entrance of the Capitol South Metro station during rush hour near the U.S. Captiol April 16, 2013 in Washington, DC. Police and other law enforcement agencies in Washington and across the country continue to operate at a heightened state of security after yesterday's Boston Marathon bombings. (Photo credit: Chip Somodevilla/Getty Images)

Security Remains Heightened In Washington Day After Bombings During Boston Marathon

WASHINGTON, DC - APRIL 16: A U.S. Capitol Police K-9 team works at the entrance of the Capitol South Metro station during rush hour near the U.S. Captiol April 16, 2013 in Washington, DC. Police and other law enforcement agencies in Washington and across the country continue to operate at a heightened state of security after yesterday's Boston Marathon bombings. (Photo credit: Chip Somodevilla/Getty Images)

Security Remains Heightened In Washington Day After Bombings During Boston Marathon

WASHINGTON, DC - APRIL 16: A U.S. Capitol Police K-9 team works at the entrance of the Capitol South Metro station during rush hour near the U.S. Captiol April 16, 2013 in Washington, DC. Police and other law enforcement agencies in Washington and across the country continue to operate at a heightened state of security after yesterday's Boston Marathon bombings. (Photo credit: Chip Somodevilla/Getty Images)